Natural rubber is known to exhibit a high level of strain-induced crystallization. Strain-induced crystallization is a phase transformation that an amorphous material undergoes when subjected to stress (strain). Generally, crystalline polymer chains are ordered (parallel) and closely packed, and amorphous polymer chains are disordered. The mechanism of strain-induced crystallization is believed to be a primary contributor to natural rubber's unique properties, including its outstanding tensile strength and good crack growth resistance. Synthetic rubbers such as polyisoprene, polybutadiene and styrene-butadiene copolymer do not inherently exhibit the high level of strain-induced crystallization that natural rubber possesses.